Illuminating light incident on tissue is transmitted through, scattered by, absorbed, or reflected by that tissue. At certain wavelengths, after absorbing the illuminating light, tissue can re-emit light energy at a different wavelength (autofluorescence). If a substance is introduced into the tissue or is present between tissue layers, or in lumens, it can fluoresce after absorbing incident light as well. Detecting devices can be placed in relationship to the tissue to image light that is transmitted, scattered, reflected, or fluoresced from the tissue. It is well known in the art that certain wavelengths of light tend to be preferentially absorbed, reflected, or transmitted through different types of tissue. Generally, near infrared light (600-1300 nm) tends to coincide with minima in the spectral absorption curve of tissue, and thus allows the deepest penetration and transmission of light. For optical analysis of surface structures or diagnosis of diseases very close to the body surface or body cavity surfaces or lumens, UV light and visible light below 600 nm can also be used, as it tends to be absorbed or reflected near the surface of the tissue.
Various modalities are currently used for imaging of tissue and organs, including visible light endoscopes, ultrasound, magnetic resonance imaging (MRI), computed tomography (CT), and positron emission tomography (PET). Many anatomical spaces and tissues, however, are not easily accessible and viewable. Moreover, the use of imaging equipment can be expensive and time consuming, and their application is often limited.
Various contrast agents are also employed to effect image enhancement in a variety of fields of diagnostic imaging, the most important of these being X-ray, magnetic resonance imaging (MRI), ultrasound imaging, and nuclear medicine. Additionally, optical labels, such as fluorescent dyes, are introduced into tissue samples to signal abnormal biological and/or chemical conditions of tissues of a living subject. Despite many successful applications, conventional optical labels have many drawbacks. For example, conventional optical labels are generally toxic to living cells and tissues comprised of living cells. Additionally, conventional optical labels such as fluorescent dyes generally suffer from short-lived fluorescence because the dyes undergo photo bleaching after minutes of exposure to an excitation light source. This renders them unsuitable for optical imaging that requires extended time period of monitoring. Moreover, conventional optical labels are sensitive to environmental changes such as pH and oxygen concentration. Another drawback of conventional optical labels is that typically the excitation spectra of such labels are quite narrow, while the emission spectra of such labels is relatively broad, resulting in overlapping emission spectra. Thus, when a combination of conventional optical labels with different emission spectra are used in optical imaging, multiple filters are need to detect the resultant emission spectra of the combination. Additionally, fluorescent labels are generally inefficient at converting the excitation light to the emission wavelength, and the resulting signal can be very weak.
Accordingly, there remains a need for improved compositions, methods, and devices for use in medical imagining, and more particularly for marking, indicating, and illuminating tissue.